Stochastic Particle Acceleration in Turbulence Generated by the Magnetorotational Instability

TL;DR

This study explores how cosmic rays are stochastically accelerated by turbulence generated by the magnetorotational instability in accretion flows, revealing anisotropic diffusion and efficient shear acceleration for energetic particles.

Contribution

It provides a detailed numerical analysis of cosmic ray acceleration in MRI-driven turbulence using shearing box simulations without back reaction effects.

Findings

Cosmic rays diffuse anisotropically with higher diffusion along the flow direction.

The momentum distribution of CRs becomes isotropic over time.

Shear acceleration is effective for high-energy particles.

Abstract

We investigate stochastic particle acceleration in accretion flows. It is believed that the magnetorotational instability (MRI) generates turbulence inside accretion flows and that cosmic rays (CRs) are accelerated by the turbulence. We calculate equations of motion for CRs in the turbulent fields generated by MRI with the shearing box approximation without back reaction to the field. The results show that the CRs randomly gain or lose their energies through the interaction with the turbulent fields. The CRs diffuse in the configuration space anisotropically: The diffusion coefficient in direction of the unperturbed flow is about twenty times higher than the Bohm coefficient, while those in the other directions are only a few times higher than the Bohm. The momentum distribution is isotropic, and its evolution can be described by the diffusion equation in momentum space where the diffusion coefficient is a power-law function of the CR momentum. We show that the shear acceleration efficiently works for energetic particles. We also cautiously note that in the shearing box approximation, particles that cross the simulation box many times along the radial direction suffer unphysical runaway acceleration by the Lorentz transformation, which needs to be taken with special care.